Google has acquired ReCaptcha, one of those companies behind the distorted text boxes at the bottom of many Web site sign-in pages.

Terms of the deal were not disclosed, but Google plans to use ReCaptcha’s technology both as a security measure within certain Google sites and to make its massive book-scanning project a little smarter, the company said in a blog post.

ReCaptcha is an offshoot of Carnegie Mellon University’s School of Computer Science, and puts a twist on the traditional captcha: a string of letters in squiggly text meant to confuse spam bots and other nonhuman Web pests.

The idea behind a captcha is to confuse a computer, but computers are also confused by some words written in fonts used long ago. ReCaptcha offers two words, one of which is a captcha it already knows, and one of which is a word it doesn’t know. The thinking is that if you get the first word right, you’re likely a human and you’re also probably going to get the second one right.

It can then pool all the answers for the second word and declare with a reasonable amount of certainty that the second word is what most people think it is, thereby updating the vocabulary of participating book scanners. This is of obvious interest to Google, currently bent on scanning as many books as it can find.